(1) Field of the Invention
The present invention relates to a novel method for crosslinking an isoprene-isobutylene rubber which is a synthetic rubber, as well as to a crosslinked rubber product obtained by the method.
(2) Description of the Prior Art
Isoprene-isobutylene rubber (hereinafter abbreviated to "butyl rubber" in some cases) is produced by copolymerization of isoprene and isobutylene, has an unsaturation degree of 0.5 to 3 mole %, and is a known synthetic rubber. Crosslinked butyl rubber has low gas permeability, electrical insulation, heat resistance, damping property, resistance to acids and alkalis, low water absorption, etc., and is in use in rubber vibration insulator, automotive tube, packing, rubber stopper, o-ring, etc.
For crosslinking of butyl rubber, there have been known three methods, i.e. sulfur crosslinking, quinoid crosslinking and resin crosslinking. Of these, resin crosslinking is most preferred in order to obtain a crosslinked butyl rubber which satisfies the requirements for heat resistance, low compression set, high hardness, high electrical insulation, low corrosivity to metals, etc.
Regarding the technique for crosslinking of butyl rubber, there is the following description in p. 117 of "POLYSAR Butyl Handbook" issued by Polysar Co. in 1996.
"POLYSAR Butyl compounds with exceptional heat resistance and low compression set can be obtained by curing with dimethylol phenol resins. Halogen-bearing activators are customarily used in conjunction with the resin, but it is possible to produce satisfactory cures without activation by curing at high temperature, especially with high unsaturation POLYSAR Butyl rubbers."
Thus, a butyl rubber becomes a crosslinked butyl rubber of good properties by adding only a dimethylphenol resin thereto and conducting crosslinking at 180 to 210.degree. C. The crosslinked butyl rubber has good properties in heat resistance, low compression set, electrical insulation and corrosivity to metals. However, when there is required a crosslinked butyl rubber having good electrical insulation and yet a high hardness, it is difficult to find an appropriate combination of raw materials enabling the production of such a crosslinked butyl rubber. That is, carbon black, which is used as a filler to impart a certain hardness to a crosslinked butyl rubber obtained, has an upper limit in the addition amount when the crosslinked butyl rubber must have electrical insulation; in such a case, therefore, use of carbon black alone is unable to allow the obtained crosslinked butyl rubber to have a desired hardness. Examples of the crosslinked butyl rubber product having a high hardness are a hard butyl rubber roller, a packing for high-pressure-water pipe and a sealing rubber for electrolytic capacitor.
In producing such a product, a silane coupling agent has been used, in addition to carbon black, to obtain a required hardness. The silane coupling agent includes silanes such as vinylsilane, mercaptosilane, aminosilane and the like. When a silane is added, the silanol group of the silane reacts with silica or clay (which is added as other filler), whereby a high strength and a high hardness are obtained; however, the hardness obtained is unstable.
For example, the hardness obtained differs depending upon whether a silane is added in a mixture with silica or clay or a silane is added separately. Further, when a closed type kneader (e.g. a Banbury mixer) is used and when a silane is added to a compound (a raw material mixture) of 150.degree. C. or higher, no stable hardness is obtained because the silane is vaporized or its reaction with silica or clay is unstable (thus, there are indefinite parameters). The reason therefor is thought to be that the reaction of the silane with silica or clay is difficult to control.
Production of a crosslinked butyl rubber of high hardness by resin crosslinking is possible by using a halogen compound (e.g. tin chloride or chloroprene rubber) in combination with a phenolic resin, when there is no upper limit to the amount of carbon black used. Use of a halogen compound, however, is not preferred depending upon the application of the crosslinked butyl rubber obtained, because the halogen compound may cause metal corrosion. Further, increase in carbon black amount for production of crosslinked butyl rubber of high hardness is not preferred, either, because the compound (raw material mixture) used for production of such a crosslinked butyl rubber has low moldability or because the crosslinked butyl rubber obtained has low electrical insulation.
Herein, "butyl rubber of high hardness" refers to a butyl rubber having a JIS-A hardness or durometer-A hardness of 80 or more.